1. Field of the Invention
The present invention relates to display devices driven with a voltage-controlled matrix of capacitive loads, such as an active-matrix liquid crystal display device, and more specifically to driving circuits and driving methods for such display devices.
2. Description of the Related Art
In portable information devices, such as mobile phones, PDAs (personal digital assistants) or notebook computers, there is a high demand for reducing power consumption in view of prolonging the battery life of the built-in battery. On the other hand, due to enhanced processing capabilities and more sophisticated use of these portable information devices, a demand has developed for high-quality display capabilities with more display colors. For this reason, the display devices used in such portable information devices should be adapted to the need for high-quality display capabilities, and TFT (thin film transistor) active-matrix liquid crystal display devices (in the following referred to as “TFT-LCD devices”) have come to be used in place of conventional passive-matrix liquid crystal display devices.
In TFT-LCD devices, a voltage corresponding to an image signal is applied as a data signal to a display region (display portion) including capacitive loads, thereby displaying an image in that display region. Since the voltage to be applied to the display region is an analog voltage, a buffer outputting an analog signal as the data signal to be applied to the display portion (below, simply referred to as “output buffer”), such as a buffer of a D/A converter generating the analog voltage from a digital video signal, needs to perform an analog operation. Therefore, a bias current corresponding to the necessary driving capability has to be supplied to that internal portion, in order to operate the output buffer. As a result, in TFT-LCD devices, the proportion of the power consumption of that driving circuit that is taken up by the power consumption of the output buffer is large. In TFT-LCD devices that are built into the above-mentioned portable information devices, a small display region (display portion) with few pixels is used, and also the horizontal scanning frequency is low, so that the proportion of the power consumption taken up by the output buffer is particularly large. Furthermore, if dot-sequential driving is performed, as in TFT-LCD devices in which the TFTs are formed with continuous grain silicon (in the following referred to as “CG silicon”), then, because of the charging and discharging of the capacitive loads in the display region, an output buffer becomes necessary that has a driving capability that is much larger than in the case of line-sequential driving. For this reason, also in dot-sequential driving-type TFT-LCD devices, the proportion of the consumption power taken up by the output buffer is particularly large.
To address this issue, JP 2002-149125A discloses a liquid crystal display device in which the number of analog buffers (output buffers) is reduced by providing for each set of a plurality of data lines one analog buffer that receives an analog signal obtained by D/A conversion of a digital signal representing the image to be displayed, and outputs a data signal (analog voltage) to be applied to the data lines of the display panel. With this liquid crystal display device, energy is saved by reducing the number of analog buffers (output buffers).
However, this energy-saving conventional technology does not reduce the power consumption of the output buffers themselves. Furthermore, this conventional technology is premised on line-sequential driving and cannot be applied to dot-sequential driving in which one output buffer is provided from the outset for a plurality of data lines.